Multi-piece solid golf ball

Abstract

The invention provides a multi-piece solid golf ball having a solid core, at least one intermediate layer and a cover. The core has a hardness which gradually increases from a core center to a core surface, with the golf ball having specific values for the hardness difference between the core center and the core surface; the hardness difference (I)-(II) in JIS-C hardness units being not more than ±2; the (hardness of intermediate layer material)−(hardness of core surface); the (initial velocity of sphere composed of core encased by intermediate layer)−(initial velocity of core); the (deflection of sphere composed of core encased by intermediate layer)/(deflection of core); and the (hardness of cover material)−(hardness of intermediate layer material).

1. A multi-piece solid golf ball comprising a solid core, at least one intermediate layer and a cover, wherein the core has a hardness which gradually increases from a core center to a core surface, the hardness difference in JIS-C hardness units between the core center and the core surface being at least 15 and, letting (I) be the calculated average value of the cross-sectional hardness measured at a position 15 mm from the core center and the cross-sectional hardness measured at the core center and letting (II) be the measured cross-sectional hardness at a position 7.5 mm from the core center, the hardness difference (I)-(II) in JIS-C hardness units being not more than +2; the intermediate layer has a material hardness and the core has a surface hardness which together satisfy the condition
   (JIS-C hardness of intermediate layer material)−(JIS-C hardness of core surface) >0;
 
 
       a sphere composed of the core encased by the intermediate layer has an initial velocity and the core has an initial velocity which together satisfy the condition
   (initial velocity of sphere composed of core encased by intermediate layer)−(initial velocity of core) ≧0;
 
 
       the sphere composed of the core encased by the intermediate layer has a deflection and the core has a deflection which together satisfy the condition
   0.80 ≦(deflection of sphere composed of core encased by intermediate layer)/(deflection of core);
 
 
       the cover is formed of a cover material composed primarily of polyurethane; and the cover material has a Shore D hardness and the intermediate layer material has a Shore D hardness which together satisfy the condition
   (Shore D hardness of cover material)−(Shore D hardness of intermediate layer material)≦0;
 
 wherein thickness of the cover (mm) and thickness of the intermediate layer (mm) satisfy the following relationship:
   cover thickness×1.2<intermediate layer thickness<cover thickness×2.5; and
 
 
 wherein the ball has a deflection and the sphere composed of the core encased by the intermediate layer has a deflection which together satisfy the condition
   0.85<(ball deflection)/(deflection of sphere composed of core encased by intermediate layer)≦0.97.
 
 
 
     
     
       2. The multi-piece solid golf ball of  claim 1 , wherein the intermediate layer is formed primarily of a resin mixture obtained by blending as essential components:
 100 parts by weight of a resin component composed of, in admixture,
 (A) a base resin of (a-1) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer blended with (a-2) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer in a weight ratio of from 100:0 to 0:100, and 
 (B) a non-ionomeric thermoplastic elastomer in a weight ratio of from 100:0 to 50:50; 
 (C) from 5 to 120 parts by weight of a fatty acid and/or fatty acid derivative having a molecular weight of from 228 to 1500; and 
 (D) from 0.1 to 17 parts by weight of a basic inorganic metal compound capable of neutralizing un-neutralized acid groups in component A and component C. 
 
 
     
     
       3. The multi-piece solid golf ball of  claim 1 , wherein the hardness difference (I)-(II) in JIS-C units is not more than ±1. 
     
     
       4. The multi-piece solid golf ball of  claim 1 , wherein the initial speed of the sphere composed of the core encased by the intermediate layer and the initial speed of the core together satisfy the condition
   (initial speed of sphere composed of core encased by intermediate layer)−(initial speed of core)≧0.2.
 
 
     
     
       5. The multi-piece solid golf ball of  claim 1 , wherein the deflection of the sphere composed of the core encased by the intermediate layer and the deflection of the core together satisfy the condition
   0.80<(deflection of sphere composed of core encased by intermediate layer)/(deflection of core)≦0.92.
 
 
     
     
       6. The multi-piece solid golf ball of  claim 1 , wherein the intermediate layer material has a Shore D hardness of from 50 to 60.